In order to use the Relational Data Access Service for
Service Data Objects,
you will need to understand some of the concepts behind SDO:
the data graph, the data object, the disconnected way of working,
the change summary, XPath and property expressions, and so on.
If you are not familiar with these ideas, you might want to look first at
the section on SDO.
In addition, the Relational DAS makes use of the PDO extension to
isolate itself from specifics of different back-end relational databases.
In order to use the Relational DAS you will need to be able to
create and pass a PDO database connection;
for this reason you might also want to take a look at
the section on PDO.

The job of the Relational DAS is to move data between the application
and a relational database. In order to do this it needs to be told
the mapping between the database entities
- tables, columns, primary keys and foreign keys -
and the elements of the SDO model
- types, properties, containment properties and so on.
You specify this information as metadata when you
construct the Relational DAS.

Overview of Operation

The first step is to call the Relational DAS's constructor,
passing the metadata that defines the mapping between
database and SDO model.
There are examples of this below.

The next step might be to call the
executeQuery()
or
executePreparedQuery()
methods on the Relational DAS, passing either a literal SQL statement
for the DAS to prepare and execute, or a prepared statement with
placeholders and a list of values to be inserted.
You may also need to specify a small amount of metadata
about the query itself, so that the Relational DAS knows
exactly what columns will be returned from the database and
in what order. You will also need to pass a PDO database connection.

The return value from
executeQuery()
or
executePreparedQuery()
is a normalised data graph containing all the data from the result set.
For a query that returns data obtained from a number of tables,
this graph will contain a number of data objects,
linked by SDO containment references.
There may also be SDO non-containment references within the data.

Once the query has been executed and the data graph constructed,
there is no need for either that instance of the the Relational DAS or
the database connection. There are no locks held on the database.
Both the Relational DAS and the PDO database connection can be
garbage collected.

Quite possibly the data in the data graph will go through
a number of modifications. The data graph can be serialised
into the PHP session and so may have a lifetime beyond just
one client-server interaction. Data objects can be created
and added to the graph, the data objects already in the graph
can be deleted, and data objects in the graph can be modified.

Finally, the changes made to the data graph can be applied
back to the database using the
applyChanges()
method of the Relational DAS. For this, another instance
of the Relational DAS must be constructed, using the
same metadata, and another connection to the database obtained.
The connection and the data graph are passed to
applyChanges().
At this point the Relational DAS examines the change summary
and generates the necessary INSERT, UPDATE and DELETE SQL statements
to apply the changes. Any UPDATE and DELETE statements are qualified
with the original values of the data so that should the data have
changed in the database in the meantime this will be detected.
Assuming no such collisions have occurred the changes will be
committed to the database. The application can then continue to work
with the data graph, make more changes and apply them, or can discard
it.

There are other ways of working with the data in the database:
it is possible to just create data objects and write them to the
database without a preliminary call to
executeQuery(),
for example. This scenario and others are explored in the
Examples
section below.

The Relational DAS requires that the SDO extension be installed.
The SDO extension requires a version of PHP 5.1, and the
Relational DAS requires a recent version that contains
an important fix for PDO. The most up-to-date information about
required levels of PHP should be found in the changelog for the
package on PECL. At the time of writing, though, the Relational
DAS requires the most recent beta level of PHP 5.1, that is
PHP 5.1.0b3.

The Relational DAS uses PDO to access the relational database,
and so should run with a variety of different relational databases.
At the time of writing it has been tested in the following configurations

MySQL 4.1.14, on Windows.
The Relational DAS operates correctly with the php_pdo_mysql
driver that comes with the pre-built binaries for PHP 5.1.0b3.

MySQL 4.1.13, on Linux. It is necessary to have the most up-to-date
PDO driver for MySQL, which comes built in to PHP 5.1.0b3.
It may be necessary to uninstall the usual driver that would have
come from PECL using
pear uninstall pdo_mysql
. You will need to configure PHP with the
--with-pdo-mysql option.

DB2 8.2 Personal Edition, on Windows.
The Relational DAS operates correctly with the
php_pdo_odbc driver that comes with the pre-built binaries for
PHP 5.1.0b3.

DB2 8.2 Personal Developer's Edition, on Linux.
The Developer's Edition is needed because it
contains the include files needed when PHP is configured
and built. You will need to configure PHP with the
--with-pdo-odbc=ibm-db2 option.

The Relational DAS applies changes to the database within
a user-delimited transaction: that is, it issues a call to
PDO::beginTransaction()
before beginning to apply changes, and
PDO::commit()
or
PDO::rollback()
on completion.
Whichever database is chosen, the database and the PDO driver
for the database must support these calls.

There are the following limitations in the current release of the
Relational DAS:

No support for nulls. There is no support for SQL NULL type.
It is not legal to assign PHP NULL to a data object property
and the Relational DAS will not write that back as a NULL to the
database. If nulls are found in the database on a query,
the property will remain unset.

Only two types of SDO relationship. The metadata described below
allows the Relational DAS to model just two types of SDO relationship:
multi-valued containment properties and single-valued
non-containment properties. In SDO, whether a property is
single- or multi-valued, and whether it is containment or
non-containment, are independent. The full range of possibilities
that SDO allows cannot all be defined. There may be relationships
that it would be useful to model but which the current implementation
cannot manage.
One example is a single-valued containment relationship.

No support for the full range of SDO data types. The Relational
DAS defines all primitive properties in the SDO model as being
of type string. SDO defines a richer set of types containing various
integer, float, boolean and data and time types. String is adequate
for the purposes of the Relational DAS since the combination of PHP,
PDO and the database will ensure that values passed as strings
will be converted to the proper type before being put in the database.
This does affect some scenarios in which the Relational DAS has
to work with a data graph that has come from or will go to a
different DAS.

Only one foreign key per table.
The metadata only provides the means to specify one foreign key
per table. This foreign key may be mapped to one of the two types of
SDO relationship supported. Obviously there are some scenarios
that cannot be described under this limitation - it is not possible
to have two non-containment references from one table to another
for example.

This section illustrates how the Relational DAS can be used to create,
retrieve, update and delete data in a relational database.
Many of the examples are illustrated with a three-table database that
contains companies, departments within those companies, and employees
that work in those departments. This example is used in a number of
places within the SDO literature. See the examples section of the
Service Data Objects specification
or the
Examples
section of the documentation for the SDO extension.

The Relational DAS is constructed with metadata that defines the
relational database and how it should be mapped to SDO.
The long section that follows describes this metadata and how to
construct the Relational DAS. The examples that follow it all assume that
this metadata is in an included php file.

The examples below and others can all be found in the
Scenarios
directory in the Relational DAS package.

The Relational DAS throws exceptions in the event that it finds errors
in the metadata or errors when executing SQL statements against the
database. For brevity the examples below all omit the use of try/catch
blocks around the calls to the Relational DAS.

These examples all differ from the expected use of SDO in two
important respects.

First, they show all interactions with the database completed within
one script. In this respect these scenarios are not realistic but are
chosen to illustrate just the use of the Relational DAS.
It is expected that interactions with the database will be separated
in time and the data graph serialised and deserialised into the PHP
session one or more times as the application interacts with an end user.

Second, all queries executed against the database use hard-coded
queries with no variables substituted. In this case it is safe to
use the simple
executeQuery()
call, and this is what the examples illustrate.
In practice, though, it is unlikely that the SQL statement is known
entirely ahead of time. In order to allow variables to be safely
substituted into the SQL queries, without running the risk of
injecting SQL with unknown effects, it is safer to use the
executePreparedQuery()
which takes a prepared SQL statement containing placeholders
and a list of values to be substituted.

This first long section describes in detail how the metadata describing
the database and the required SDO model is supplied to the
Relational DAS.

When the constructor for the Relational DAS is invoked, it needs to be
passed several pieces of information. The bulk of the information,
passed as an associative array in the first argument to the constructor,
tells the Relational DAS what it needs to know about the relational
database. It describes the names of the tables, columns, primary keys
and foreign keys. It should be fairly easy to understand what is
required, and once written it can be placed in a php file and included
when needed. The remainder of the information, passed in the second
and third arguments to the constructor, tells the Relational DAS what
it needs to know about the relationships between objects and the shape
of the data graph; it ultimately determines how the data from the
database is to be normalised into a graph.

The first argument to the constructor describes the target
relational database.

Each table is described by an associative array with up to four keys.

Key

Value

name

The name of the table.

columns

An array listing the names of the columns, in any order.

PK

The name of the column containing the primary key.

FK

An array with two entries, 'from' and 'to', which define
a column containing a foreign key, and a table to which the foreign
key points. If there are no foreign keys in the table then the
'FK' entry does not need to be specified. Only one foreign key
can be specified. Only a foreign key pointing to the primary key
of a table can be specified.

Note that although in this example there are no foreign keys specified
to the database and so the database is not expected to enforce
referential integrity, the intention behind the
co_id
column on the department table and the
dept_id
column on the employee table is they should contain the primary key
of their containing company or department record, respectively.
So these two columns are acting as foreign keys.

There is a third foreign key in this example, that from the
employee_of_the_month
column of the company record to a single row of the employee table.
Note the difference in intent between this foreign key and the other
two. The
employee_of_the_month
column represents a single-valued relationship: there can be only
one employee of the month for a given company.
The
co_id
and
dept_id
columns represent multi-valued relationships: a company can contain
many departments and a department can contain many employees.
This distinction will become evident when the remainder of the metadata
picks out the company-department and department-employee relationships
as containment relationships.

There are a few simple rules to be followed when constructing the
database metadata:

All tables must have primary keys, and the primary keys must be
specified in the metadata. Without primary keys it is not possible
to keep track of object identities. As you can see from the SQL
statements that create the tables, primary keys can be
auto-generated, that is, generated and assigned by the database when
a record is inserted. In this case the auto-generated primary key
is obtained from the database and inserted into the data object
immediately after the row is inserted into the database.

It is not necessary to specify in the metadata all the columns
that exist in the database, only those that will be used.
For example, if the company table had another column that the
application did not want to access with SDO, this need not be
specified in the metadata. On the other hand it would have done
no harm to specify it: if specified in the metadata but never
retrieved, or assigned to by the application, then the unused column
will not affect anything.

In the database metadata note that the foreign key definitions
identify not the destination column in the table which is pointed
to, but the table name itself. Strictly, the relational model
permits the destination of a foreign key to be a non-primary key.
Only foreign keys that point to a primary key are useful for
constructing the SDO model, so the metadata specifies the table name.
It is understood that the foreign key points to the primary key of
the given table.

Given these rules, and given the SQL statements that define the
database, the database metadata should be easy to construct.

The Relational DAS uses the database metadata to form most of the
SDO model. For each table in the database metadata, an SDO type
is defined. Each column which can represent a primitive value
(columns which are not defined as foreign keys) are added
as properties to the SDO type.

All primitive properties are given a type of string in the SDO model,
regardless of their SQL type. When writing values back to the
database the Relational DAS will create SQL statements that treat
the values as strings, and the database will convert them to the
appropriate type.

Foreign keys are interpreted in one of two ways, depending on the
metadata in the third argument to the constructor that defines
the SDO containment relationships.
A discussion of this is therefore deferred until the section on
SDO containment references
below.

The second argument to the constructor is the application root type.
The true root of each data graph is an object of a special root type
and all application data objects come somewhere below that. Of the
various application types in the SDO model, one has to be the
application type immediately below the root of the data graph.
If there is only one table in the database metadata, the application
root type can be inferred, and this argument can be omitted.

The third argument to the constructor defines how the types in the
model are to be linked together to form a graph. It identifies the
parent-child relationships between the types which collectively form a
graph. The relationships need to be supported by foreign keys to be
found in the data, in a way shortly to be described.

The metadata is an array containing one or more associative arrays,
each of which identifies a parent and a child. The example below shows
a parent-child relationship from company to department, and another
from department to employee. Each of these will become an SDO
multi-valued containment property in the SDO model.

Foreign keys in the database metadata are interpreted as either
multi-valued containment properties or single-valued non-containment
properties, depending on whether they have a corresponding SDO
containment reference specified in the metadata. In the example here,
the foreign keys from department to company (the
co_id
column in the department table)
and from employee to department (the
dept_id
column in the employee table) are interpreted as supporting the
SDO containment references.
Each containment reference mentioned in the SDO containment references
metadata must have a corresponding foreign key present in the
database and defined in the database metadata. The values of the
foreign key columns for containment references do not appear in the
data objects, instead they are represented by the containment reference
from the parent to the child. So the
co_id
column in the department row in the database, for example, does not
appear as a property on the department type, but instead as a
containment relationship called
department
on the company type.
Note that the foreign key and the parent-child relationship appear to
have opposite senses: the foreign key points from the department to
the company, but the parent-child relationship points from company to
department.

The third foreign key in this example, the
employee_of_the_month
,
is handled differently.
This is not mentioned in the SDO containment references metadata.
As a consequence this is interpreted in the second way: it becomes
a single-valued non-containment reference on the company object, to
which can be assigned references to SDO data objects of the employee
type. It does appear as a property on the company type. The way to
assign a value to it in the SDO data graph is to have a graph that
contains an employee object through the containment references, and to
assign the object to it. This is illustrated in the later examples
below.

The following set of examples all use the Relational DAS to work with
a data graph containing just one application data object, a single
company and the data just to be found the company table. These examples
do not exercise the power of SDO or the Relational DAS and of course
the same result could be achieved more economically with direct SQL
statements but they are intended to illustrate how to work with the
Relational DAS.

For this very simple scenario it would be possible to simplify the
database metadata to include just the company table - if that were done
the second and third arguments to the constructor and the column
specifier used in the query example would become optional.

Пример 1. Creating a data object

The simplest example is that of creating a single data object and
writing it to the database. In this example a single company object
is created, its name is set to 'Acme', and the Relational DAS is
called to write the changes to the database. The company name is
set here using the property name method. See the
Examples
section on the SDO extension for other ways of accessing the
properties of an object.

Data objects can only be created when you have a data object to
start with, however. It is for that reason that the first call
to the Relational DAS here is to obtain a root object. This is
in effect how to ask for an empty data graph - the special root
object is the true root of the tree. The company data object is
then created with a call to
createDataObject()
on the root object. This creates the company data object and inserts
it in the graph by inserting into a multi-valued containment property
on the root object called 'company'.

When the Relational DAS is called to apply the changes a simple
insert statement 'INSERT INTO company (name} VALUES ("Acme");'
will be constructed and executed. The auto-generated primary key
will be set into the data object and the change summary will be reset,
so that it would be possible to continue working with the same data
object, modify it, and apply the newer changes a second time.

/************************************************************** * Get a database connection and write the object to the database ***************************************************************/$dbh = new PDO(PDO_DSN,DATABASE_USER,DATABASE_PASSWORD);$das -> applyChanges($dbh, $root);?>

Пример 2. Retrieving a data object

In this example a single data object is retrieved from the database
- or possibly more than one if there is more than one company
called 'Acme'. For each company returned, the
name
and
id
properties are echoed.

In this example the third argument to
executeQuery(),
the column specifier is needed as there are other tables in the
metadata with column names of
name
and
id.
If there were no possible ambiguity it could be omitted.

/************************************************************** * Construct the DAS with the metadata ***************************************************************/$das = new SDO_DAS_Relational ($database_metadata,'company',$SDO_reference_metadata);

/************************************************************** * Get a database connection ***************************************************************/$dbh = new PDO(PDO_DSN,DATABASE_USER,DATABASE_PASSWORD);

/************************************************************** * Issue a query to obtain a company object - possibly more if they exist ***************************************************************/$root = $das->executeQuery($dbh,'select name, id from company where name="Acme"', array('company.name', 'company.id') );

This example combines the previous two, in the sense that in order
to be updated the object must first be retrieved. The application
code reverses the company name (so 'Acme' becomes 'emcA') and then the
changes are written back to the database in the same way that they
were when the object was created. Because the query searches for
the name both ways round the program can be run repeatedly to find
the company and reverse its name each time.

In this example the same instance of the Relational DAS is reused
for the
applyChanges(),
as is the PDO database handle. This is quite alright; it also
alright to allow the previous instances to be garbage collected
and to obtain new instances. No state data regarding the graph
is held the Relational DAS once it has returned a data graph to
the application. All necessary data is either within the graph itself,
or can be reconstructed from the metadata.

/************************************************************** * Construct the DAS with the metadata ***************************************************************/$das = new SDO_DAS_Relational ($database_metadata,'company',$SDO_reference_metadata);

/************************************************************** * Get a database connection ***************************************************************/$dbh = new PDO(PDO_DSN,DATABASE_USER,DATABASE_PASSWORD);

/************************************************************** * Issue a query to obtain a company object - possibly more if they exist ***************************************************************/$root = $das->executeQuery($dbh,'select name, id from company where name="Acme" or name="emcA"', array('company.name', 'company.id') );

/************************************************************** * Alter the name of just the first company ***************************************************************/$company = $root['company'][0];echo "obtained a company with name of " . $company->name . "\n";$company->name = strrev($company->name);

/************************************************************** * Construct the DAS with the metadata ***************************************************************/$das = new SDO_DAS_Relational ($database_metadata,'company',$SDO_reference_metadata);

/************************************************************** * Get a database connection ***************************************************************/$dbh = new PDO(PDO_DSN,DATABASE_USER,DATABASE_PASSWORD);

/************************************************************** * Issue a query to obtain a company object - possibly more if they exist ***************************************************************/$root = $das->executeQuery($dbh,'select name, id from company where name="Acme" or name="emcA"', array('company.name', 'company.id') );

/************************************************************** * Delete any companies found from the data graph ***************************************************************/unset($root['company']);

The following set of examples all use two tables from the company
database: the company and department tables. These examples exercise
more of the function of the Relational DAS.

In this series of examples a company and department are created,
retrieved, updated, and finally deleted. This illustrates the
lifecycle for a data graph containing more than one object. Note that
this example clears out the company and department tables at the start
so that the exact results of the queries can be known.

You can find these examples combined into one script called
1cd-CRUD
in the
Scenarios
directory in the Relational DAS package.

Пример 5. One company, one department - Create

As in the earlier example of creating just one company data object,
the first action after constructing the Relational DAS is to call
createRootDataObject()
to obtain the special root object of the otherwise empty data graph.
The company object is then created as a child of the root object,
and the department object as a child of the company object.

When it comes to applying the changes, the Relational DAS has to
perform special processing to maintain the foreign keys that support
the containment relationships, especially if auto-generated primary
keys are involved. In this example, the relationship between the
auto-generated primary key
id
in the company table and the
co_id
column in the department table must be maintained. When inserting a
company and department for the first time the Relational DAS has to
first insert the company row, then call PDO's
getLastInsertId()
method to obtain the auto-generated primary key, then add that as
the value of the
co_id
column when inserting the department row.

/*************************************************************** Create a company with name Acme and one department, the Shoe department***************************************************************/$dbh = new PDO(PDO_DSN,DATABASE_USER,DATABASE_PASSWORD);$das = new SDO_DAS_Relational ($database_metadata,'company',$SDO_reference_metadata);

$root = $das -> createRootDataObject();

$acme = $root -> createDataObject('company');$acme -> name = "Acme";

$shoe = $acme->createDataObject('department');$shoe->name = 'Shoe';

$das -> applyChanges($dbh, $root);

?>

Пример 6. One company, one department - Retrieve and Update

In this case the SQL query passed to
executeQuery()
performs an inner join to join the data from the company
and department tables. Primary keys for both the company and
department tables must be included in the query. The result set
is re-normalised to form a normalised data graph. Note that a
column specifier is passed as the third argument to the
executeQuery()
call enabling the Relational DAS to know which column is which in
the result set.

Note that the
co_id
column although used in the query is not needed in the result set.
In order to understand what the Relational DAS is doing when it builds
the data graph it may be helpful to visualise what the result set
looks like. Although the data in the database is normalised, so that
multiple department rows can point through their foreign key to one
company row, the data in the result set is non-normalised: that is,
if there is one company and multiple departments, the values for the
company are repeated in each row. The Relational DAS has to reverse
this process and turn the result set back into a normalised data graph,
with just one company object.

In this example the Relational DAS will examine the result set and
column specifier, find data for both the company and department
tables, find primary keys for both, and interpret each row as
containing data for a department and its parent company. If it has
not seen data for that company before (it uses the primary key to
check) it creates a company object and then a department object
underneath it. If it has seen data for that company before and
has already created the company object it just creates the
department object underneath.

In this way the Relational DAS can retrieve and renormalise data
for multiple companies and multiple departments underneath them.

/************************************************************** * Retrieve the company and Shoe department, then delete Shoe and add IT ***************************************************************/$dbh = new PDO(PDO_DSN,DATABASE_USER,DATABASE_PASSWORD);$das = new SDO_DAS_Relational ($database_metadata,'company',$SDO_reference_metadata);

$acme = $root['company'][0]; // get the first company - will be 'Acme'$shoe = $acme['department'][0]; // get the first department underneath - will be 'Shoe'

unset($acme['department'][0]);

$it = $acme->createDataObject('department');$it->name = 'IT';

$das -> applyChanges($dbh, $root);?>

Пример 7. One company, two departments - Retrieve and Delete

In this example the company and department are retrieved and
then deleted. It is not necessary to delete them individually
(although that would be possible) - deleting the company object
from the data graph also deletes any departments underneath it.

Note the way that the company object is actually deleted using the
PHP unset call. The unset has to be performed on the containing
reference which in this case is
the company reference on the special
root object. You must use:

<?phpunset($root['company'][0]);?>

and not:

<?phpunset($acme); //WRONG?>

Simply unsetting
$acme
would destroy the variable but leave the data in the data
graph untouched.

/************************************************************** * Retrieve the company and IT department, then delete the whole company ***************************************************************/$dbh = new PDO(PDO_DSN,DATABASE_USER,DATABASE_PASSWORD);$das = new SDO_DAS_Relational ($database_metadata,'company',$SDO_reference_metadata);

The following examples use all three tables from the company database:
the company, department, and employee tables. These introduce the final
piece of function not exercised by the examples above: the
non-containment reference
employee_of_the_month.

Like the examples above for company and department, this set of examples
is intended to illustrate the full lifecycle of such a data graph.

Пример 8. One company, one department, one employee - Create

In this example a company is created containing one department and
just one employee. Note that this example clears out all three tables
at the start so that the exact results of the queries can be known.

Note how once the company, department and employee have been created,
the
employee_of_the_month
property of the company can be made to point at the new employee.
As this is a non-containment reference, this cannot be done until
the employee object has been created within the graph.
Non-containment references need to be managed carefully.
For example if the employee were now deleted from under the department,
it would not be correct to try to save the graph without
first clearing or re-assigning the
employee_of_the_month
property.
The closure rule for SDO data graphs requires that any object pointed
at by a non-containment reference must also be reachable by
containment references.

When it comes to inserting the graph into the database, the procedure
is similar to the example of inserting the company and department,
but
employee_of_the_month
introduces an extra complexity.
The Relational DAS needs to insert the objects working down the tree
formed by containment references, so company, then department, then
employee. This is necessary so that it always has the auto-generated
primary key of a parent on hand to include in a child row. But when
the company row is inserted the employee who is employee of the month
has not yet been inserted and the primary key is not known. The
procedure is that after the employee record is inserted and its
primary key known, a final step is performed in which the the
company record is updated with the employee's primary key.

/************************************************************************************** Create a tiny but complete company.* The company name is Acme.* There is one department, Shoe.* There is one employee, Sue.* The employee of the month is Sue.*************************************************************************************/$das = new SDO_DAS_Relational ($database_metadata,'company',$SDO_reference_metadata);$dbh = new PDO(PDO_DSN,DATABASE_USER,DATABASE_PASSWORD);

Пример 9. One company, one department, one employee - Retrieve and update

The SQL statement passed to the Relational DAS is this time an inner
join that retrieves data from all three tables. Otherwise this example
introduces nothing that has not appeared in a previous example.

The graph is updated by the addition of a new department and employee
and some alterations to the name properties of the existing objects
in the graph. The combined changes are then written back. The
Relational DAS will process and apply an arbitrary mixture of
additions, modifications and deletions to and from the data graph.

/************************************************************************************* * Find the company again and change various aspects. * Change the name of the company, department and employee. * Add a second department and a new employee. * Change the employee of the month. *************************************************************************************/$das = new SDO_DAS_Relational ($database_metadata,'company',$SDO_reference_metadata);$dbh = new PDO(PDO_DSN,DATABASE_USER,DATABASE_PASSWORD);

$acme->employee_of_the_month = $billy;$das -> applyChanges($dbh, $root);echo "Wrote back company with extra department and employee and all the names changed (Megacorp/Footwear/Susan)\n";

?>

Пример 10. One company, two departments, two employees - Retrieve and delete

The company is retrieved as a complete data graph containing five
data objects - the company, two departments and two employees.
They are all deleted by deleting the company object. Deleting an
object from the graph deletes all the object beneath it in the graph.
Five SQL DELETE statements will be generated and executed. As always
they will be qualified with a WHERE clause that contains all of the
fields that were retrieved, so that any updates to the data in the
database in the meantime by another process will be detected.

/************************************************************************************* * Now read it one more time and delete it. * You can delete part, apply the changes, then carry on working with the same graph but * care is needed to keep closure - you cannot delete the employee who is eotm without * reassigning. For safety here we delete the company all in one go. *************************************************************************************/$das = new SDO_DAS_Relational ($database_metadata,'company',$SDO_reference_metadata);$dbh = new PDO(PDO_DSN,DATABASE_USER,DATABASE_PASSWORD);

You may be interested in seeing the SQL statements that are generated
in order to apply changes back to the database. At the top of the
SDO/DAS/Relational.php
you will find a number of constants which control whether the process
of constructing and executing the SQL statements is to be traced.
Try setting
DEBUG_EXECUTE_PLAN
to TRUE to see the generated SQL statements.

The Relational DAS provides two classes: the Relational DAS itself and
the subclass of Exception that can be thrown. The Relational DAS has
four publicly useful calls: the constructor,
the
createRootDataObject()
call to obtain the root object of an empty data graph,
the
executeQuery()
call to obtain a data graph containing data from a relational database,
and the
applyChanges()
call to write changes made to a data graph back to the relational
database.

SDO_DAS_Relational::executePreparedQuery --
Executes an SQL query passed as a prepared statement, with a
list of values to substitute for placeholders, and return the
results as a normalised data graph.